1. Field of the Invention
The invention relates to gas chromatography and more particularly, to a gas chromatograph in which a material sample is conductable through a first separating device and subsequently conductable through a second separating device by a carrier gas.
2. Description of the Related Art
DE 28 06 123 C2 discloses one type of gas chromatograph. Here, by setting different pressure gradients between auxiliary gas paths of a switching device and between these paths and connection points of a main gas path, it is possible either to allow the components of a material sample emerging from a first separating device (i.e., separation column or separation column circuit) to enter into the second separating device or, for the operating mode (i.e., “cutting”) to block entry. Here, it is possible, in the operating mode, for the components to be fed by the appropriate auxiliary gas path to a downstream detector or a third separating device. Moreover, the switching device can be used to back-flush the first separating device with a carrier gas. The switching device that is required for switching over the gas flows comes into contact only with the carrier gas, but the switching device does not come into contact with the material sample. The main, auxiliary and connecting gas paths are in the form of a multi-part construction involving capillaries, but this is relatively complex, especially as the parts have to be adjusted relative to one another.
WO 00/17634 A2 discloses a switching device structure that is considerably simpler and more planar.
DE 28 06 123 C2 and WO 00/17634 A2 show different ways of setting the pressure in the auxiliary gas paths of the switching device, but at least one dedicated pressure regulator is required in addition to the a dedicated pressure regulator for regulating the pressure of the carrier gas introduced into the gas chromatograph.
Thus, in the case of the switching device depicted in FIG. 1 of DE 28 06 123 C2, both auxiliary gas paths contain a dedicated pressure regulator, where the first auxiliary gas path additionally contains a flow resistor in the form of a needle valve and a controllable solenoid valve in parallel with the controllable solenoid valve downstream of the outlet of the associated pressure regulator. Pressure switching in the auxiliary gas paths can also be performed without the two valves, simply by adjusting the set point of the pressure regulator in the first auxiliary gas path to a higher or lower value than the value set at the pressure regulator in the second auxiliary gas path. Manual setting of the different pressures is performed iteratively in a number of steps and is correspondingly complex. If the pressure of the carrier gas introduced into the first separating device is changed in order, for example, to optimize the separation rate (i.e., measuring rate) or separation performance, the pressures to be supplied to the auxiliary gas paths by the pressure regulators have to be reset each time. In addition, each pressure regulator requires a constant load flow for regulation, which leads to a high consumption of carrier gas.
In the case of the switching device depicted in FIG. 2 of DE 28 06 123 C2, both auxiliary gas paths together have a dedicated common pressure regulator. A needle valve with a controllable solenoid valve in parallel therewith is arranged in the first auxiliary gas path, downstream of the outlet of the pressure regulator. The second auxiliary gas path likewise contains a needle valve downstream of the outlet of the associated pressure regulator. Here, manual adjustment of the needle valves to set the different pressures in the auxiliary gas paths is complex and also occurs iteratively in a number of steps. In addition, the flow resistance of needle valves is temperature-dependent and, not least for this reason, it is very difficult to set precise pressure drops accurately by needle valves. With each change in the pressure of the carrier gas introduced into the first separating device, the pressures in the auxiliary gas paths have to be reset.